The IP (Internet Protocol) Multimedia Core Network Subsystem (referred to as IMS for short) is an IP-based network architecture proposed by the 3rd Generation Partnership Project (referred to as 3GPP for short), and it constructs an open and flexible service environment that supports multimedia applications and can provide a wide variety of multimedia services for users.
In the IMS service architecture, the control layer is separated from the service layer, and the control layer does not provide particular services, but only provides essential functions such as triggering, routing, charging, etc. The service triggering and control function in the control layer is implemented by the call session control function (referred to as CSCF for short, and divided into three categories: Proxy, Interrogating and Serving, wherein the Serving takes main responsibilities, and Interrogating is optional), and the service layer consists of a series of application servers (referred to as AS for short) and can provide particular services, wherein the AS can be a separate entity or situated in the S-CSCF (Service Call Session Control Function). The control layer (S-CSCF) controls service triggering according to customer subscription information and invokes services on the ASs to implement service functions. The AS and S-CSCF are collectively called server equipments (SE). The end-to-end equipments in a session are called user equipments (UE), in charge of the interaction with the users. These functional entities use the Session Initiation Protocol (SIP) to communicate. In the services required to play multimedia information, the participation of the media resource function (referred to as MRF for short) is required; the MRF is divided into a control part MRFC and a processing part MRFP, and the MRFC is in charge of translating and interpreting the SIP signaling of a SE into instructions for the MRFP, or reversely, translating and interpreting a response from the MRFP into SIP signaling to be sent to the SE, while the MRFP is in charge of sending and receiving a media stream.
When a multimedia session is established between two terminals, if one of them disconnects the session, the session termination process is entered, and for users, this process seems dull and boring. On the other hand, at present, if a user intends to acquire some information, such as the balance of the prepaid phone expense, a session has to be established specially for this interrogation, which is not convenient.